Re: SSI tuning points

The attached patch addresses one of the open non-blockers for beta3.

These are tuning points which emerged in testing. The first is more
likely to be helpful. The second may be very important in a few
types of transaction mixes, but I threw in a lot of weasel words and
qualifiers because someone could easily try this to bring down the
transaction retry rate, but suffer a net loss in throughput because
less efficient plans could be chosen. I hope I made that point in a
reasonable fashion, although I'm certainly open to suggestions for
better wording.

-Kevin

On Fri, Jun 17, 2011 at 5:50 PM, Kevin Grittner
<Kevin(dot)Grittner(at)wicourts(dot)gov> wrote:
> The attached patch addresses one of the open non-blockers for beta3.
>
> These are tuning points which emerged in testing.  The first is more
> likely to be helpful.  The second may be very important in a few
> types of transaction mixes, but I threw in a lot of weasel words and
> qualifiers because someone could easily try this to bring down the
> transaction retry rate, but suffer a net loss in throughput because
> less efficient plans could be chosen.  I hope I made that point in a
> reasonable fashion, although I'm certainly open to suggestions for
> better wording.

This is good advice, but I think it could use a bit more wordsmithing.
How about something like this:

When the system is forced to combine multiple page-level predicate
locks into a single relation-level predicate lock because the
predicate lock table is short of memory, an increase in the rate of
serialization failures may occur. You can avoid this by increasing
max_pred_locks_per_transaction.

A sequential scan will always necessitate a table-level predicate
lock. This can result in an increased rate of serialization failures.
It may be helpful to encourage the use of index scans by reducing
random_page_cost or increasing cpu_tuple_cost. Be sure to <etc.>

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company